The present invention relates to a cardiac pacemaker. In the normal healthy individual the output of blood by the heart is varied continuously to meet the metabolic demands of the body. The cardiac output changes with variations in the heart rate and the volume of blood pumped at each heart beat. The cardiac output is the product of these two variables. During strenuous exercise a four or five fold increase in cardiac output may be required. Approximately 75% of this increase is achieved by elevation of the heart rate.
Patients with cardiac disease resulting in very slow heart rates frequently require treatment in the form of an implanted artificial pacemaker. This consists of a small electrical pulse generator that is connected to the heart by an insulated electrode lead, usually positioned in the right ventricle.
Until recently the majority of such pacemakers were relatively simple devices that provided a constant frequency of stimulation. These suffer from the disadvantage that they are unable to increase the heart rate during exercise, thereby limiting the degree of exertion that the patient is able to undertake. This drawback has led to the development of two forms of pacemaker that can increase stimulation rate during exercise. The "ideal" system is a dual chamber pacemaker which has a second lead implanted in the right atrial chamber of the heart. This detects the rate of the heart's natural pacemaker and enables the artificial pacemaker to provide normal physiological changes in heart rate. However, such systems are not suitable for many patients and also have inherent technical and economic disadvantages.
An alternative approach that has been developed during recent years is the single chamber rate-responsive pacemaker. The fundamental principles of this device are as follows:
(A) A sensor is used to detect changes in a biological variable that are directly or indirectly proportional to the level of metabolic activity in the body. Evoked QT interval, respiratory rate, mixed venous temperature and body vibration are the main variables that are sensed by currently available rate-responsive pacemakers.
(B) The output from the sensor is processed by electronic circuitry to provide a signal that can be used to vary the stimulation rate provided by the pacemaker. This circuitry is designed to perform a series of mathematical and logical functions that are known collectively as the rate-response algorithm.
The two principal functions of the rate-responsive algorithm are:
(i) To determine the recognition level within the pacemaker for the value of the sensor signal which must be exceeded in order for an increase in pacing rate to be initiated (threshold level).
(ii) To determine the precise mathematical relationship between changes in the sensor signal S and the change in pacing rate P to be provided by the pacemaker, (the slope dP/dS).
Although every individual requires substantially the same heart rate as other individuals for a given level or type of physical activity, the changes which take place in the sensed biological variable as a result of a given level of physical activity differ from individual to individual. Accordingly, conventional rate response pacemakers are arranged so that the values of the threshold and slope may be adjusted to suit the individual patient. These values can be selected by telemetered instructions from an external programmer. The programmer is also used to select the minimum and maximum rates that the pacemaker will generate and to vary a number of other aspects of pacemaker function such as pulse amplitude and duration.
Conventional rate responsive pacemakers utilise a single biological sensor to modulate the pacing rate. Studies on the clinical performance of such pacemakers have revealed that no single sensor system is able to accurately reproduce the changes in heart rate that are seen in healthy individuals during the performance of normal daily activities. Thus pacing systems that detect movement of the body (e.g. by sensing physical vibrations) are able to provide correctly timed changes in hear rate but do not necessarily provide a response that is proportional to the degree of exercise that the user is undertaking. Pacing systems that indirectly detect changes in metabolic activity (e.g. by sensing the rate of respiration or the temperature of mixed venous blood) are able to generate changes in heart rate that are more physiologically appropriate in terms of their magnitude, but these responses may take place so slowly that they are too late to be of haemodynamic benefit to the user.
European Patent Application 249820 discloses an attempt at solving these problems. The pacemaker disclosed in that document includes several sensors each for sensing a respective different physiological variable and a selector circuit which successively selects different ones of the signals and/or combinations thereof in predetermined time steps after the start of an exercise cycle. In the example given, an acceleration sensor determines the start of an exercise period and controls the pacing rate during the initial portion of the exercise period which is predetermined and selected to be within the range 10 to 60 seconds. During the middle period of the exercise, which is predetermined and selected to be within the range 30 seconds to 20 minutes, a respiration sensor determines pacing rate. In the final period of the exercise which is also predetermined and selected to be within the range 30 seconds to 3 minutes, pacing rate continues to be determined by the respiration sensor. Although the European patent application also indicates that the selected signal and/or combination of signals may also be dependent upon other factors such as predetermined pacing rates, those output signals which fluctuate least, or the falling below a threshold of a single output signal or combination thereof, there is no specific description of what these dependencies might be in practice.
The present inventors consider that a pacemaker constructed as described in European Application 249820 would, in practice, suffer from disadvantages. For example, during the initial predetermined time period of the exercise, namely from 10 to 60 seconds, pacing rate varies with the magnitude of the signal from the accelerometer and thus, during this period, due to the deficiencies in such sensors, the actual pacing rate may differ substantially from the most appropriate pacing rate. For example, a person running downstairs may produce substantial vibration which would cause the accelerometer to produce a large output signal which in turn would cause the pacemaker to produce a higher pacing rate than actually required whereas a person running upstairs may produce less vibration, resulting in a lower pacing rate than actually required. Further, the predetermining of the lengths of the time periods in an exercise cycle during which different sensor signals are selected will mean that, in practice, where an individual undertakes a variety of different exercise cycles, the switching from one sensor signal to another may take place at inappropriate times.
Hertzschrittmacher, 6, 1986 pages 64 to 67 contains a report of some studies carried out into the possibility of providing a pacemaker with two sensors, one sensing bodily vibrations and the other central venous blood temperature. This study proposes that the advantages of the two systems can be obtained by utilizing the combination of such sensors and proposes that the pacing frequency increase should result from either a blood temperature increase or a change in the measured activity. In effect, therefore, this system would select a pacing rate equal to the higher of the rates indicated by the two sensors. In view of the inaccurate relationship between the magnitude of the signal output by a vibration sensor and required pacing rate, this system would not be satisfactory.
EP 259658 (Intermedics Inc) discloses a further attempt at solving the problem that vibration sensors, whilst responding rapidly to bodily vibrations, do not provide an accurate indication of the pacing rate required whereas sensors which sense such changes in metabolic activity as rate of respiration or mixed venous blood temperature respond slowly but, after response, may provide a signal which accurately indicates an appropriate pacing rate. EP 259658 suggests that filtering the output of a vibration sensor to suppress frequencies above approximately 4 Hz will provide a signal which not only responds rapidly to changes in bodily vibration but also has a "direct and substantially linear relationship" with the work being performed by the patient. The pacing rate is determined by an algorithm which utilises such a signal and a signal from a second sensor sensing, for example, blood temperature. This algorithm involves establishing a number of successively higher thresholds for the amplitude of the filtered signal from the vibration sensor and a number of fixed predetermined successively higher pacing rates associated with each threshold respectively. When the amplitude of the filtered signal from the vibration sensor exceeds a given threshold, the pacing rate is increased (at a predetermined rate of increase) to the respective value and counting of a predetermined time-out period is begun. If the signal from the second sensor (e.g. blood temperature sensor) increases within the time-out period to a level indicating that the required pacing rate is higher than the predetermined pacing rate associated with the relevant threshold, further increase in the pacing rate takes place in accordance with the level of the signal from the second sensor. If, however, the signal from the second sensor fails to increase to such a level within the time-out period, it is assumed that the increase in the filtered vibration sensor signal has been in error and a fault recovery program is executed to reduce the pacing rate back to a base level.
The algorithm disclosed in EP 259658 is also such that if the signal from the second sensor increases without the amplitude of the filtered signal from the first sensor having exceeded its first threshold some increase in pacing rate may take place under control of the signal from the second sensor, but this increase is limited to a small value. The description in EP 259658 appears to envisage that, under conditions where the amplitude of the filtered signal from the first sensor has exceeded a given one of the thresholds, the further increase in pacing rate which may take place under control of the second signal (provided it has increases sufficiently within the time out period) is limited to a small value so that the higher pacing rates can only be achieved if the filtered vibration sensor signal first exceeds successively higher thresholds.
Regardless of whether filtering out frequencies above 4 Hz in a signal derived from a vibration sensor would actually provide a signal substantially linearly related to the work being performed by a patient as suggested in EP 259658, the proposal in this patent suffers from a number of problems.
Firstly, the setting up of even a single threshold is a relatively complicated and skilled operation requiring the patient to undergo selected exercises whilst a skilled technician or physician monitors the operation of the pacemaker in order to enable him to program in an appropriate threshold. The setting of a number of successively high thresholds, therefore, would be extremely complex and time consuming and the present inventors believe that it may in fact be impossible to achieve a satisfactory set of such thresholds in the majority of patients. Further complexity arises in the need for setting time-out periods and the need for defining the small amounts of increase in pacing rate which may take place if the second signal increases without the amplitude of the filtered vibration sensor signal having first exceeded an appropriate threshold.
Secondly, the algorithm disclosed in EP 259658 is such that a number of common activities could not be adequately provided for. One example is any form of exercise that generates significant body vibrations but does not require high levels of energy expenditure, e.g. brushing one's teeth. Such a form of exercise will generate high amplitude vibrations in the frequency range of 1-4 Hz. These will be detected by the activity sensor and cause a rapid rise in pacing rate as a number of vibration threshold levels are exceeded. The complementary sensor will not indicate the need for a pacemaker rate response and will eventually ensure that the pacemaker rate returns to near base levels. However, this decline in rate will not occur until the termination of the time out period, and will thus not prevent a period of excessive pacing rate occurring which may last for two or three minutes, for example. Another example is any form of exercise that requires a high level of energy expenditure by the patient but does not generate high amplitude vibration signals, e.g. swimming. In this situation it is likely that only a lower vibration threshold level will be exceeded, resulting in a small increase in pacemaker rate. The rate will of course increase further during sustained exertion when the ongoing energy expenditure is detected by the complementary sensor, but it appears that the algorithm of EP 259658 will limit the magnitude of this secondary rise in pacemaker rate if the vibration signal remains small.